Evaluation of the mechanical alignment of a linear accelerator with an electronic portal imaging device (EPID)

Mechanical misalignment of a medical linear accelerator can be caused by any combination of source position displacement relative to the collimator rotational axis, collimator jaw asymmetry, or when the rotational axes of the gantry and the collimator do not intersect. A test procedure sensitive to all of these problems has been developed using an Electronic Portal Imaging Device (EPID). Each marker is placed on top of the EPID housing and on the treatment couch, then two images are acquired for gantry positions 180 degrees apart. By comparing the positions of the markers and their distances to the beam centre, mechanical alignment of the linear accelerator can be assessed. By comparing the positions of the beam centre for another two images acquired at collimator angles 180 degrees apart, the three potential sources of misalignment can be distinguished. Results with Siemens' Beam View were presented and determination accuracy of better than 0.25 mm can be achieved.